Saponins form a large family of naturally occurring glycoconjugate compounds with considerable structural diversity. To the steroid, triterpenoid or steroidal alkaloid aglycone in these compounds a variable number of sugars is attached by the glycosidic bonds. The saponins display a broad spectrum of biological activities and practical applications. Their beneficial pharmaceutical activities have been applied inter alia as:    1) Absorption adjuvants in pharmaceutical compositions. For example, U.S. Pat. No. 4,501,734 describes the use of a triterpenoid saponin extract from Sapindus mukurossi Gaertn. to increase absorption of coadministered β-lactam antibiotic.    2) Immunological adjuvants in vaccine compositions against a variety of diseases. The saponins typically used as immunological adjuvants are triterpene glycosides extracted from the Quillaja saponaria, e.g., U.S. Pat. No. 5,057,540, WO 91/04052; similar application was described for the saponins Quinoa, pat. appl. WO 96/03998.    3) Anti-inflammatory, e.g., aescin, a saponin from Aesculus hippocastanum seeds; U.S. Pat. No. 5,118,671.    4) Anti-ulcerous agent, e.g., Glyccyrrhiza glabra saponins; U.S. Pat. No. 5,166,139.    5) Anti-cancer agents, e.g., a composition consisting of steroidal and triterpenoid saponins found in plants including Quillaja saponaria Malina, pat. appl. US 2005/0175623; OSW-1 saponin-patent CN 1951394, WO 2004/091484, US 2005/004044).
The saponin OSW-1, 3β16β17α-trihydroxycholest-5-en-22-one 16-O-{2-O-(4-methoxybenzoyl)-β-D-xylopyranosyl-(1→3)-2′-O-acetyl-α-L-arabinopyranoside (formula I) belongs to a family of cholestane glycosides, isolated from the bulbs of Ornithogalum saundersiae by Japanese scientists in 1992 (Kubo et al. Phytochemistry, 31, 3969, 1992).

In NCI tests for leukemia HL-60 cancer cells the saponin OSW-1 exhibited cytotoxic activity in nanomolar concentrations, i.e. a cytotoxicity about 10-100 times higher than that of clinically applied anticancer agents, such as mitomycin C, adriamycin, cisplatin, camptothecin, and taxol. In the initial in vivo trials the saponin OSW-1 appears to prolong the life span of mice bearing P388 by 59% after only a single administration of 0.01 mg/kg. The effectiveness of OSW-1 in in vivo tests on mouse model of human cancer was demonstrated also by American researchers in the pat. appl. WO 2004/091484.
The saponin OSW-1 exhibits a unique mechanism of action. Its profile of cytotoxic activity does not match any of the six known mechanisms of action (alkylating agents, topoisomerase I inhibitors, topoisomerase II inhibitors, antimetabolites of RNA/DNA, antimetabolites of DNA, antimitotic agents). The first report concerning the OSW-1 mechanism of action was published in 2005. It was revealed that OSW-1 damaged the mitochondrial membrane and cristae in both human leukemia and pancreatic cancer cells, leading to the loss of transmembrane potential, increase of cytosolic calcium and activation of calcium-dependent apoptosis (Zhou et al., J. Natl. Cancer Inst. 97, 1781, 2005; Zhu et al., Mol. Pharmacol. 68, 1831, 2005).
So far several methods of OSW-1 synthesis have been described (patents: CN 101029070, CN 1844138, U.S. Pat. No. 6,753,414; publications: Deng et al., J. Org. Chem. 64, 202, 1999; Guo and Fuchs, Tetrahedron Lett. 39, 1099, 1998; Yu and Jin, Am. Chem. Soc. 123, 3369, 2001; ibid. 124, 6576, 2002; Morzycki and Wojtkielewicz, Carbohydr. Res. 337, 1269, 2002; Xu et al., Tetrahedron Lett. 44, 9375, 2003; Liu et al., Org. Chem. 73, 157, 2008; Tsubuki et al., Tetrahedron Lett. 49, 229, 2008). However, only two methods avoid the use of toxic and expensive OsO4 in the crucial step of the OSW-1 aglycone synthesis. One of them was proposed by an American group in 2002 (U.S. Pat. No. 6,753,414). In the second method, elaborated by our group, the desired trans diol in ring D was obtained by the cleavage of the corresponding epoxide with LiOH/H2O2 (Morzycki et al., Tetrahedron 57, 2185, 2001; Morzycki and Wojtkielewicz, Carbohydr. Res. 337, 1269, 2002; patent PL 191517 B1).
A highly potent anticancer activity, selectivity towards malignant tumor cells and unique mechanism of action make the saponin OSW-1a promising novel anticancer agent. The application of this compound or its analogues as anticancer drugs was described in patents: CN 1951394, WO 2004/091484, US 2005/004044. Synthesis of OSW-1, because of its relatively complicated structure, consists of several steps and usually is not very efficient. Therefore, synthesis of analogues having a simplified structure, but retaining a high and selective activity was attempted. So far, a large number of OSW-1 analogues with modified aglycone or sugar was obtained and their cytostatic activity was tested (patents: CN 1010899008, CN 101029072, WO 2005/082924; publications: Guoet al., Bioorg. Med. Lett., 9, 419, 1999; Ma et al., Carbohydr. Res. 329, 495, 2000; Ma et al., Carbohydr. Res. 334, 159 2001; Ma et al., Bioorg. & Med. Chem. Lett. 11, 2153, 2001; Den et al., J. Chem. 22, 994, 2004; Morzycki et al., Bioorg. Med. Chem. Lett. 14, 3323, 2004; Deng et al., Bioorg. Med. Chem. Lett. 14, 2781, 2004; Matsuya et al., Eur. J. Org. Chem. 797, 2005; Shi et al., J. Org. Chem. 70, 10354, 2005; Tang et al., Bioorg.& Med. Chem. Lett. 17, 1003, 2007; Peng etr al., Bioorg.& Med. Chem. Lett. 17, 5506, 2007; Tschamber et al., Bioorg.& Med. Chem. Lett. 17, 5101, 2007; Wojtkielewicz et al., J. Med. Chem. 50, 3667, 2007).
A new series of derivatives of the saponin OSW-1 that are the object of the present invention are useful for selective inhibition of cell division cycle and induction of apoptosis in cancer cells. This group of new saponin derivatives is capable of selectively damaging the mitochondrial membrane and mitochondrial activity, thus allowing to achieve a very strong anticancer properties, particularly against leukaemia, pancreatic and melanoma cancers. Hence, they can be used as antimitotic and pro-apoptotic drugs, particularly as anticancer drugs. Furthermore, the new mechanism of action of the compounds provided by this invention promises their potential application as effective agents in the treatment of cancers resistant to conventional anticancer drugs.